The present invention refers to a steam or hot water boiler comprising a convection part surrounding a flame tube, and a final combustion chamber forming an extension of the flame tube, but located outside the convection part, the latter being formed as a cylindrical drum having plane end plates, within which at least two groups of smoke tubes are arranged concentrically around the flame tube, and the water cooled wall of the final cobustion chamber is connected to the adjacent end plate of the drum, between two groups of smoke tubes.
The aim of the invention is to heat, in a simple and inexpensive manner, a boiler fluid, above what is ordinarily obtainable directly in a convection part. A further aim is to promote circulation of the water within the convection part, while simultaneously, in a steam boiler, protect the portion of the flame tube passing through a steam dome located within the convection part.